The present invention relates generally to prosthetic hip implants and, more particularly, to an adaptor for attaching a spherical head component to a femoral stem component.
Conventional prosthetic hip joints generally include a femoral component having a spherical head member affixed to the femur by means of a femoral stem secured within the femur. In the case of total hip replacement, a so-called standard femoral head is used and is adapted to be received within an acetabular cup that is inserted into the acetabulum of the hip socket. In the case of a hip replacement that does not require a separate acetabular cup, an endoprosthesis (endo) head is utilized and mates directly with the acetabulum. Since endo heads mate directly with the acetabulum, they have a larger spherical diameter than standard femoral heads used with prosthetic acetabular cups.
Standard femoral heads are produced in a variety of neck lengths (i.e. "short", "medium", and "long") to provide optimal prosthesis-to-patient fit, where "short", "medium", and "long" relate to the center of rotation of the head relative to the stem taper. In contrast, endo heads are typically produced having the same location for the center of rotation with respect to the hip stem taper, i.e., the same neck length, and this location is independent of the spherical size of the head. Accordingly, an interpositional plug or head adaptor is required in those cases in which it is necessary to increase the neck length for such an endo head.
When performing hip replacement surgical procedures, the physician has available many different varieties of femoral head components and stem components in a variety of different sizes. Essentially, the physician must select the size necessary to form a joint that matches the physiology of the patient's hip. Once a particular femoral stem component is selected, the proper head with the appropriate neck length must be selected. In the case of total hip replacements, the physician chooses the standard femoral head component having the desired neck length.
Problems could arise when a head adaptor is available to provide an extended neck length for an endo head because the endo head adaptor could be inadvertently assembled to a standard size head. This would be a particular problem if the endo head adaptor was assembled to a standard head which already had a built-in extended neck length and therefore too long of a neck would be provided. In addition, the endo head adaptor may not be suitably seated within and sufficiently supported by the smaller standard head, thereby causing excessive pressures to be exerted on the thin wall of the endo head adaptor. Eventually, the adaptor will wear prematurely and must be replaced.